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48 lines
2.0 KiB
XML
48 lines
2.0 KiB
XML
<!DOCTYPE qhelp PUBLIC
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"-//Semmle//qhelp//EN"
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"qhelp.dtd">
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<qhelp>
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<overview>
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<p>The <code>for</code> statement is designed to allow you to iterate over the elements of a
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sequence or other iterable object. Strings in Python are iterable, and often used as such.
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However, they are also often considered, not as sequences of characters, but as atomic entities.
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</p>
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<p>
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One source of defects in Python is mistakenly iterating over a non-iterable object such as an integer.
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This sort of defect is easily detected as a <code>TypeError</code> will be raised. However, if a string
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is mistakenly used as the iterable in a <code>for</code> statement, which also receives other sequences
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(such as lists) then the code will iterate over the string one character at a time.
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This is probably not what the programmer intended and results in errors that are hard to find.
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</p>
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</overview>
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<recommendation>
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<p>Since this defect usually indicates a logical error, it is not possible to give a general method
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for addressing the defect. However, adding a guard that checks that the iterator is not a string
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could be worthwhile.
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</p>
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</recommendation>
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<example>
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<p>
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In this example, the loop may iterate over <code>"Hello"</code> producing one character per line,
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as well as over <code>[ "Hello", "World" ]</code>
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It is likely that the programmer forgot to wrap the <code>"Hello"</code> in brackets.
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</p>
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<sample src="IterableStringOrSequence.py" />
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</example>
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<references>
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<li>Python Language Reference: <a href="http://docs.python.org/reference/compound_stmts.html#the-for-statement">The for statement</a>,
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<a href="http://docs.python.org/2.7/reference/datamodel.html#object.__iter__">object.__iter__</a>.</li>
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<li>Python Standard Library: <a href="http://docs.python.org/dev/library/stdtypes.html#iterator-types">Iterator types</a>.</li>
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<li>Scipy lecture notes: <a href="http://scipy-lectures.github.io/advanced/advanced_python/#iterators">Iterators,
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generator expressions and generators</a>.</li>
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</references>
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</qhelp>
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